📝 update docs

Author: acontreras89

src/ScrollSync.js

@@ -9,6 +9,11 @@ import PropTypes from 'prop-types'
 export default class ScrollSync extends Component {
 
   static propTypes = {
+    /**
+     * Callback to be invoked any time synchronization happens
+     *
+     * @param {Element} el The element that has received the scroll event
+     */
     onSync: PropTypes.func,
     children: PropTypes.element.isRequired,
     proportional: PropTypes.bool,